The present invention relates to an oxide cathode, and more particularly, to an oxide cathode having improved electron emission characteristics and a longer lifetime.
The schematic structure of an oxide cathode will be explained referring to the attached FIG. 1.
The oxide cathode is provided with a circular tube type sleeve 2 which supports a cap-type metal base 1 in which nickel (Ni) is contained as a main component and small amounts of silicon (Si), magnesium (Mg), etc. are contained as a reducing agent and which houses a heater 3 for heating the cathode, and an electron emissive material layer 4 which is coated and formed on the metal base 1 containing barium (Ba) as a main component and acting as an electron emission source during cathode operation. That is, the oxide cathode is manufactured by closing up an end of a hollow, circular tube type sleeve with a metal base, inserting a heater in the sleeve for heating the cathode, and forming an electron emissive material layer of a mixture of two, three or more alkaline earth compounds on the surface of the metal base.
The electron emissive material layer of the oxide cathode is manufactured as follows.
First, complex carbonate particles of alkaline earth compounds containing barium are dispersed in an organic solvent containing binder and then the thus obtained dispersion is coated on a metal base such as nickel (Ni), platinum (Pt) containing reducing agents by a spraying or electrodeposition method. Thereafter, the coated layer is thermally decomposed to a complex oxide of alkaline earth compounds and aged to an electron emittable state to produce free barium through the reaction of the oxide with reducing agents contained in the metal base.
The electron emissive material layer which emits thermoelectrons is formed on the metal base as an oxide layer of an alkaline earth metal. As for the alkaline earth metal oxides, initially, oxides containing barium were employed. However, two-component oxides with strontium or three-component oxides with strontium and calcium are now widely employed as the homogeneous mixture, as the two- and three-component oxides are known to have good characteristics. These alkaline earth metal oxides absorb carbon dioxide or moisture from the air and react with them to give alkaline earth metal carbonates or hydroxides. That is, the oxides are unstable in an ambient, atmosphere, so alkaline earth metal salts (for example carbonate) of two- or three-component mixture-type which are stable in an ambient atmosphere are used. A dispersion of the metal salts in water or in an organic solvent is sprayed, electrodeposited or coated on the metal base to form a layer and then the metal salts are decomposed to form an oxide layer by the heater installed inside in a vacuum while removing gases using a vacuum pump.
The cathode having the electron emissive material layer is assembled in an electron tube, and heated to about 1000.degree. C. by the heater during an evacuating process to make a vacuum. At this time, the metal salts, for example barium carbonate, decomposes to barium oxide as follows. EQU BaCO.sub.3 .fwdarw.BaO+CO.sub.2 .uparw. (1)
The thus-obtained barium oxide is reduced by the reducing agents such as silicon and magnesium at the interface with the metal base during cathode operation as follows. EQU BaO+Mg.fwdarw.MgO+Ba.uparw. (2) EQU 4BaO+Si.fwdarw.Ba.sub.2 SiO.sub.4 +2Ba.uparw. (3)
The produced free barium contributes to the electron emission. At this point, compounds such as MgO and Ba.sub.2 SiO.sub.4 are produced at the interface of the electron emissive material layer and metal base as described in formulae (2) and (3). The product accumulates and forms a barrier, (a so-called "interlayer") at the interface, and this barrier interrupts diffusion of Mg or Si and makes the free barium production difficult. Therefore, this interlayer contributes to the shortening of the cathode lifetime and other undesirable results. Moreover, this interlayer has high resistance, and current density is limited because the interlayer disturbs the electron emission current flow.
The oxide cathode is widely used as an electron emission source for an electron tube since the manufacture thereof is easy and the characteristics thereof are good. However, the large and fine electron tubes require enhanced characteristics of electron emission and a longer lifetime. Accordingly, research to improve operation current density of the oxide cathode and lengthen the lifetime are continuously carried out.
Among the various factors which determine the lifetime of a cathode, the reduction of the barium content accompanied by the cathode operation or the interlayer growth as described above act as important factors. Hence, research for improving the cathode lifetime as well as electron emission ability by changing electron emission components or including specific compounds therein have been carried out.
Japanese Patent Laid-open sho 63-254635 discloses that the lifetime of a cathode manufactured by including indium compounds such as indium carbonate, indium oxide, indium hydroxide, and organic compounds of indium in a three-component carbonate can be enhanced about 1.5 times with respect to the cathode manufactured by employing a three-component carbonate at 0.5A/cm.sup.2.
However, the above-mentioned cathode has certain drawbacks in that the time required for aging is at least twice that required in the conventional cathode, and the initial characteristic is rather lower than that of the conventional cathode.